1. Field of the Invention
The present invention relates to methods that can be used to protect series bus type connection circuits against the consequences of an accidental short circuit occurring on such a bus. It also relates to the devices that can be used to implement such a device.
2. Description of the Prior Art
It is known that, to set up a network of instruments or devices communicating with one another in the field of "smart" buildings, home automation systems, data processing, robotics and automobiles, it is often the practice to use bus type links such that far less attention need be paid to the location of the devices connected to these links. To this end, a set of wires is used, forming a cable that serves the different points at which the communicating devices are likely to be placed, and each communicating device is connected, in a by-pass or branch connection, to these wires by means of a connector enabling the transmission and/or reception of the signals on the wires, including signals pertaining to the supply of the communicating part, Different arrangements, such as a hierarchically organized system for example, enable each device connected to the bus to exchange messages and or information elements of a general nature and to sort out the messages and/or information elements intended for it from all those flowing through the bus, The connectors and the messages and/or information elements sent out in the form of voltage and/or current pulses must meet one or more transmission and reception protocols governed by a certain number of standards designed to enable exchanges among a wide variety of devices that have possibly come, in particular, from different suppliers abiding by the same standard.
The different standards may be classified by category according to different criteria. Thus, there is the known category relating to buses wherein, for example with a series bus, the information flows bit after bit from one piece of equipment to another. This makes it possible then to use a transmission circuit formed by two wires only, known as a "twisted pair".
Given this series transmission, the physical nature of the cable, and the use of a relatively high level of current amounting to some tens of milliamperes to transmit the signals, the transmission speed of such a bus is limited to some Mbits per second. However hand, the equipment used is inexpensive.
In fact, the most commonly used cable comprises four wires, forming what is called a "quad cable" and the remaining two free wires are often used to transmit the supply current for certain devices connected to the bus. Furthermore, the signals are generally transmitted on the two data wires symmetrically with respect to the ground and, in this case, the voltages representing these signals appear as complementary voltages on both wires. These wires are then generally called "DATA" and "(DATA)" (for typographical reasons, brackets are used here instead of the upper bar). When these two data wires are looped by reception circuits having the planned impedance, the same current flows in both wires but in reverse directions, and the signals are therefore complementary as described further above.
It is being planned, at present, to use circuits of this type in applications known such as home automation or computerized domestic applications which include the control of domestic appliances in a dwelling as well as the control of the different accessories of an automobile. In both cases, the links must be provided with a high degree of security, even though the linking circuits to the devices thus connected have to be very moderately priced and are therefore reliable only to a very relative degree, and even though, furthermore, they are subjected to frequently rough environmental conditions and to mishandling by unprofessional users. The installation most usually comprises a central processing unit wherein the essential part of the intelligence of the system is concentrated, and decentralized units corresponding to modular functions working under the control of the central processing unit.
It is a relatively common occurrence that one of these decentralized wires should suffer a malfunction and that this malfunction should cause a short circuit in the data wires. The problem may arise also in the supply wires but it is then easily resolved by a known circuit-breaking device such as a fuse and/or an electronic device.
For the data wires, it is difficult to implement the fuse type of approach and, in this case, the primary consequence of a short circuit is the total interruption of the links between the central processing unit and the other decentralized units and hence a general breakdown of the entire system. Furthermore, if this situation persists, the end result may be that the connection circuits to the bus, which are located in the central processing unit, go out of operation. Indeed, these circuits then let through a high level of current that could amount to several tens of amperes or even more and, even if these circuits include self-protection devices, the problem of locating and reporting the faulty product is not resolved for all that.